Due to the high loads that fifth wheel coupling assemblies between tractors and trailers must bear, and the fact that they are subject to continued sliding action, fifth wheel couplings commonly require repeated lubrication. Metal bearing surfaces of fifth wheel couplings tend to wear badly and eventually must be replaced.
To reduce friction and wear, bearing plates of the type utilized for fifth wheel couplings and for other purposes such as side plates for conveyor belts may be made from extremely durable polymeric materials such as ultra high molecular weight polyethylene (hereafter, "UHMWPE") have been proposed. Reference is made to Colwell, U.S. Pat. No. 5,431,424, Kaim, U.S. Pat. No. 5,165,714 and Cork, U.S. Pat. No. 5,620,770. In use, UHMWPE bearing plates adapted for use in fifth wheel assemblies are subjected to substantial stresses as mating bearing surfaces of fifth wheel assemblies are brought together with substantial force. The primary stresses which bearing plates of this type must withstand are compression and shear forces that are imposed on bearing plates as a tractor and trailer are coupled together and as the tractor is steered into a turn. Similarly, the stationary upright side bearing plates utilized on the sides of a conveyor are subjected to shear and compression forces and can wear badly particularly when abrasive materials are being conveyed.
UHMWPE plates can be fastened to the underlying steel plate of a fifth wheel assembly only with some difficulty. As disclosed in U.S. Pat. No. 5,620,770, couplings of this type can utilize screw fasteners as shown in FIG. 4 of that reference, or can employ adhesives as shown in FIG. 5. When screw fasteners are used, screw holes are first formed through the thickness of the UHMWPE plate and are countersunk so that the head of the screw is beneath the upper surface of the bearing plate. As the bearing plate becomes worn in use, however, the screws eventually become exposed and can damage the fifth wheel coupling. Moreover, machining screw holes through the thickness of an UHMWPE plate tend to further weaken the plate. Adhesives in general have not been successful in strongly bonding UHMWPE to steel plates. Ultra high molecular weight polyethylene is chemically rather inert, and bonds to other materials only with difficulty. The same problems arise when UHMWPE bearing plates are attached to support plates by screws for other uses, such as the side walls of a conveyor.
Ultra high molecular weight polyethylene is a difficult material to form. It is commonly molded in the form of plates or rods, and parts then are machined from these bulk shapes. This is due at least in part to the difficulty that has been experienced in the past in molding UHMWPE into intricate shapes and forms.
It would be desirable to provide a UHMWPE bearing plate which could readily be fastened to a metal support plate without requiring holes to be drilled in it and in a manner that avoids the need for adhesives and also avoids the need for metal screw fasteners.